High strength porous polytetrafluoroethylene product having a coarse microstructure

ABSTRACT

Porous polytetrafluoroethylene materials having high strength and coarse microstructure are produced by densifying the materials after removal of lubricant and then stretching. The term, &#34;coarse,&#34; is used to indicate that the nodes are larger, the fibrils are longer, and the effective pore size is larger than conventional materials of the same matrix tensile strength. Densification can be achieved through the use of such devices as a densification die, a calender machine, or a press. This invention can be used to produce all kinds of shaped articles.

This is a continuation of application Ser. No. 416,465 filed Sept. 10,1982 abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

This invention relates to porous polytetrafluoroethylene (hereinafter"PTFE") materials having a unique and useful combination of highstrength and coarse microstructure, and a method for producing thesematerials. Articles made from these materials are particularly suitablefor use in the medical field.

2. Description of the Prior Art:

The products of this invention derive from paste formed products ofPTFE. Paste extrusion or paste forming techniques are old in the art andconsist of mixing a coagulated dispersion of polytetrafluoroethyleneresin with a liquid lubricant and forcing the mixture through anextrusion die or otherwise working the lubricated mixture to form acoherent shaped article. The lubricant is then removed, usually bydrying, to form a porous, unsintered PTFE article having a densityusually within the range of 1.4 to 1.7 gm/cc. Such densities correspondto porosities of 39% to 26%, respectively. At this stage, the articlecan be raised above its crystalline melt point of about 345° C. tosinter it, coalescing the porous material to form a non-porous sinteredarticle.

Alternatively, the unsintered article can be made more porous andstronger by stretching according to techniques taught in U.S. Pat. No.3,953,566. Subsequent to stretching, the stretched article can be heldrestrained and heat treated above the crystalline melt point. In thisinstance, the article remains porous and when cooled a strong porousarticle of PTFE is obtained. In the discussions which follow, the term"sintering" is used interchangeably with the process step of raising theunsintered article above its crystalline melting point. U.S. Pat. No.3,953,566 provides a method of producing all kinds of microporousstretched PTFE, such as films, tubes, rods, and continuous filaments.The articles are covered by U.S. Pat. No. 4,187,390. The microstructureof these articles consists of nodes interconnected by fibrils.

The key element of the U.S. Pat. No. 3,953,566 process is rapidstretching of PTFE. Rapid stretching allows the unsintered article to bestretched much farther than had previously been possible while at thesame time making the PTFE stronger. The rapid stretching also produces amicrostructure which is very fine in scale having, for example, a verysmall effective pore size. U.S. Pat. No. 3,962,153 describes very highlystretched products, stretch amounts exceeding 50 times the originallength. The products of both the U.S. Pat. Nos. 4,187,390 and 3,962,153have relatively high matrix tensile strengths. (See discussion of"matrix tensile strengths" and relation to article tensile strength anddensity in U.S. Pat. No. 3,953,566 at col. 3, lines 28-43.)

To compute the matrix tensile strength of a porous specimen, one dividesthe maximum force required to break the sample by the cross sectionalarea of the porous sample, and then multiplies this quantity by theratio of the density of the PTFE polymer component divided by thedensity of the porous specimen. The density of PTFE which has never beenraised above its crystalline melt point is 2.30 gm/cc while the densityof PTFE which has been sintered or raised above its crystalline meltpoint may range from above 2.0 gm/cc to below 2.30 gm/cc. For purposesof calculating matrix tensile strength in examples which follow, we haveused a density of the PTFE polymer of 2.20 gm/cc for products which havebeen raised above the crystalline melt point, and a density of 2.30gm/cc for those which have not.

When the unsintered articles are stretched at slower rates, eitherlimited stretching occurs because the material breaks, or weak materialsare obtained. These weak materials have microstructures that are coarserthan articles that are stretched equivalent amounts but at faster ratesof stretch. The term, "coarse," is used to indicate that the nodes arelarger, the fibrils are longer, and the effective pore size is larger.Such coarse microstructures would have further utility if they werestrong instead of weak.

SUMMARY OF THE INVENTION

The invention described herein teaches the manufacture of coarse, highlyporous articles of PTFE which are strong and have microstructures ofrelatively large nodes interconnected by relatively long fibrils ascompared to prior art products. Such microstructures are desired in manyinstances, and particularly in the biological field where themicrostructure must be large enough to allow cellular ingrowth andincorporation of body tissue. The key process element of the inventiondescribed herein is densification of the unsintered PTFE article afterremoval of lubricant and prior to stretching.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of the microstructure of the PTFEmaterial of the present invention.

FIG. 2 is a photomicrograph of the PTFE material of the presentinvention.

FIG. 3 is a diagram which shows a characteristic range of the coarsenessindex and matrix tensile strength obtained by densifying prior tostretching, and a characteristic range in the prior case of notdensifying prior to stretching.

FIG. 4 is a photomicrograph of the surface of prior art PTFE materialthat was stretched in one direction.

FIG. 5 is a photomicrograph of the surface of PTFE material of thepresent invention that was stretched in one direction.

FIG. 6 is a photomicrograph of the cross-section of prior art PTFEmaterial of FIG. 4.

FIG. 7 is a photomicrograph of the cross-section of PTFE material ofFIG. 5.

FIG. 8 is a photomicrograph of the surface of prior art PTFE materialthat was biaxially stretched.

FIG. 9 is a photomicrograph of the surface of PTFE material of thepresent invention that was biaxially stretched.

FIG. 10 is a schematic cross-section of the type of densification dieused in Example 3.

FIGS. 11A and 11B are light microscopy photographs of histologicalsections through filaments made in accordance with the recent inventionand according to the prior art, respectively, showing collagen ingrowth.

FIGS. 12A and 12B are photomicrographs of the filaments of FIGS. 11A and11B, respectively.

FIGS. 13A and 13B are photomicrographs of other filaments made inaccordance with the present invention and according to the prior art,respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A fully densified unsintered article of PTFE is one in which there is novoid space and such an article has a density of 2.30 gm/cc. Whenstretched under the same conditions, it is found that articles whichhave been densified to near this limit prior to stretching showdramatically coarser structures than articles which have not beendensified. There is an increasing effect with increasing densification.The highest densifications produce the most dramatic effect. In order toachieve the highest densification, it is necessary that the densifiedarticle be subjected to compressive forces until all void closure isachieved. At a fixed temperature, increased compressive forceaccelerates the rate of densification, as would be expected. For a givencompressive force, densification will occur faster at highertemperatures in the range of 300° C. than it will at lower temperatures.Less force may be required to achieve densification at highertemperatures. Higher temperatures, therefore, may facilitate thedensification inasmuch as less time and/or less compressive force may berequired. However, for otherwise identical conditions, it appears thatequivalent stretched articles are obtained independent of whetherdensification occurs at low temperatures or at high temperatures as longas equivalent densifications are achieved. It appears that the onlysignificant variable is the actual densification achieved as measured bythe density of the densified article prior to stretching.

Experiments described herein show that when densification conditions areused that result in sintering the material, the material may not be ableto be uniformly stretched. Partial sintering is known to occur below345° C. The conditions that cause sintering, therefore, establish theupper useful limit for the densification temperature.

Densification can be performed through the use of presses, dies, orcalendering machines. The use of a calendering machine to densify thedry PTFE enables the manufacture of long lengths of film.

The preferred conditions for densification in a die appear to involvepulling the material through the die at relatively low rates. The forceexerted to pull the material through the die may result in stretchingthe material that has exited the die. Lower rates require less force topull the material through the die which results in less stretching ofthe material. It appears to be desirable to minimize stretching out ofthe die. Stretching is better controlled in process steps specificallydesigned to stretch the material.

A number of processing steps can be performed prior to densification,such as calendering with the lubricant present and stretching with orwithout the lubricant present. These steps may increase the strength ofthe final article, but again, such preferred processes have not beendetected. Further, it may be preferred to not fully densify the materialprior to stretching. It is believed that the densification can beachieved by applying compressive forces in any or all directions andthat stretching can subsequently be performed in any or all directionsto yield the benefits of this invention.

It is beleived that all prior art processes specific to producing porousPTFE articles can be used in conjunction with the present invention.

While the fibril lengths and node dimensions are particularlyappropriate characteristics for identifying coarse microstructures, theypresent some problems in quantification. This arises because there is adistribution of node sizes and a distribution of fibril lengths in anygiven microstructure. Also, somewhat different microstructures areobtained depending on whether the article has been uniaxially stretched,biaxially stretched, or sequentially stretched first in one directionfollowed by stretching in a second direction. An idealized drawing ofthe node-fibril structure for the case of uniaxial stretch of a film isshown in FIG. 1. The actual electron micrograph of 198 magnification forthis structure is shown in FIG. 2.

Articles of the present invention have larger nodes and longer fibrilsthan prior art materials of similar matrix tensile strength. The fourcharacteristic dimensions of the microstructure are: node height, nodewidth, node length, and fibril length. See FIG. 1 for the definition ofthese dimensions of nodes 2 and fibrils 1 for uniaxially stretchedfilms. Fibril length 3 and node width 4 are measured in the direction ofstretching. Node length 6 is measured in the width direction of thefilm; that is orthogonal to the direction of stretching, in the plane ofstretching. Node height 5 is measured in the thickness direction of thefilm; that is, orthogonal to the plane of stretching. The distinctionbetween node width and node length may not be obvious for filmsstretched in more than one direction, since the fibrils may be orientedin many directions and the nodes may be of the same size in more thanone direction. In this case, node width is defined as the node dimensionin the same direction as the longest fibrils, in the plane ofstretching. Node height is measured in the thickness direction of thefilm; that is, orthogonal to the plane of stretching. The distinctionbetween node length and node height may not be obvious for articles witha symmetrically shaped cross-section, such as circular rods, filaments,and articles with a square cross-section. In this case, node height andnode length are said to be the same dimension termed "node height" andthis dimension is measured in the direction orthogonal to stretching.

The combination of measurements of two microstructure dimensions andstrength in the strongest direction can be used to distinguish betweenarticles of this invention and prior art articles. The combination ofthe ratio of average node height to average node width, in addition tothe average matrix tensile strength in the strongest direction, isuseful for characterizing articles of the present invention. Articles ofthis invention that have been sintered have a node height to node widthratio greater than or equal to about 3, and a matrix tensile strengthgreater than or equal to about 15,000 psi.

For materials that have been biaxially stretched, or stretched first inone direction followed by stretching in a second direction, there issome difficulty in precisely quantifying the geometry of the node-fibrilstructure. Materials that have been stretched in more than one directionhave a greater range of distribution of microstructure dimensions. Forthis reason, coarseness has also been defined in terms of otherproperties and particularly in terms of the ethanol bubble point (EBP),which is a measure of the maximum pore size in the test specimen (seeASTM F316-80). Specifically, the EBP is the minimum pressure required toforce air through an ethanol-saturated article of this invention.Raising the pressure slightly should produce steady streams of bubblesat many sites. Thus, the measurements are not biased by artifacts suchas puncture holes in the material. Ethanol bubble point is inverselyrelated to pore size; lower values of EBP indicate larger pores, or inthe terminology of this application, coarser structure. It is believedthat EBP can be assumed to be independent of the length of the path thatthe air travels through the article. In other words, it is believed thatEBP provides a characterization of pore size that is not unacceptablydependent on the dimensions of the tested article.

Another indicator of coarse structure is relatively low resistance tothe passage of air (Gurley number). Gurley number is defined as the timein seconds for 100 cc of air to flow through one square inch of materialfor a pressure of 4.9 inches of water across the material. See ASTMD-726-58 for a method of measuring the Gurley number.

In order to provide a basis for comparison of coarseness for articlesthat have been densified to different densities and subsequentlystretched, a "coarseness index" is defined here as the density of thestretched porous article divided by the EBP of that article. Density isan indicator of pore volume. Should two articles be of the same density,the article with the lower EBP is said to be coarser. Should twoarticles have the same pore size, the article with the higher density issaid to be coarser. Thus, the coarseness index is directly proportionalto density and inversely proportional to EBP. An increase in coarsenessis indicated by an increase in the coarseness index. Introducing thedensity in combination with EBP provides a means of comparing prior artarticles with articles of this invention over a wide range of matrixtensile strengths.

Sintering a restrained stretched article does lower the EBP of thearticle, and usually increases the coarseness index. However, in somecases the coarseness index may not increase due to sintering since thedensity of the article may be lowered by sintering.

FIG. 3 presents a graph of the variables, coarseness index and matrixtensile strength. Articles not heretofore available are produced withthe present invention to have a matrix tensile strength greater than orequal to about 3,000 psi and have a corresponding coarseness indexgreater than or equal to the value on a line connected by the points A,B, C, and D. The coordinates of these points are as follows: Point A[3,000 psi, 0.40 (gm/cc)/psi], Point B [12,000 psi, 0.40 (gm/cc)/psi],Point C [16,000 psi, 0.20 (gm/cc)/psi], and Point D [25,000 psi, 0.20(gm/cc)/psi].

Examples are not given for films processed at stretch ratios exceedingabout 4:1 in a direction. Higher stretch ratios generally result inarticles with higher matrix tensile strengths, as described in U.S. Pat.No. 3,953,566. No evidence exists to suggest that films of the presentinvention cannot be stretched further to obtain higher strength whilestill maintaining coarser structures than prior art films of the samestrength. It is expected that processing films at higher stretch ratioswill certainly yield films of this invention with matrix tensilestrengths exceeding 25,000 psi.

Points in the region corresponding to the present invention were derivedfrom data presented in the examples that follow. The EBP and matrixtensile strength measurements were performed subsequent to sintering therestrained, stretched articles. The conditions of sintering aredescribed in the examples. The matrix tensile strength value used wasthe value corresponding to the strongest direction of the material. Thisrepresentation of coarseness and strength is useful for characterizingmaterials that are stretched in one or more directions prior to orsubsequent to sintering.

The representation of coarseness index and strength in FIG. 3 isspecific to unfilled porous PTFE articles. Porous PTFE articles may befilled with substances such as asbestos, carbon black, pigments, andmica, as taught by U.S. Pat. Nos. 3,953,566 and 4,096,227. Articles ofthe present invention can be likewise filled. The presence of a filler,however, may affect the measurement of coarseness index since EBP is afunction of the surface tension of the porous article and the filler mayaffect the surface tension of the article.

Articles of the present invention, therefore, can be characterized inseveral ways. Either coarseness index or the node height to node widthratio, in conjunction with matrix tensile strength in the strongestdirection, can be used to describe the same products of the presentinvention. That is, coarseness index and the node height to node widthratio are not independent parameters; both describe the structure ofarticles of the present invention. Coarseness index is particularlyuseful for describing the structure of thin films in which maximum nodeheight is limited by the thickness of the film. The node height to nodewidth ratio is particularly useful for describing the structure ofarticles too small to enable the measurement of the EBP. In many cases,either of these parameters can be used to describe the structure of thesame articles.

Films of this invention that have strengths of similar magnitude inorthogonal directions can be distinguished from prior art films by thecharacterization of matrix tensile strength in orthogonal directions andEBP. This characterization pertains to sintered films having the ratioof matrix tensile strengths in orthogonal directions within the range of0.4 to 2.5, where the weaker direction has a matrix tensile strengthgreater than or equal to about 3000 psi. Films of this invention thatsatisfy these strength requirements have an EBP less than or equal toabout 4 psi.

Unsintered articles extruded from preferred resins can be stretchedfarther and more uniformly to achieve stronger stretched products thanunsintered articles extruded from non-preferred resins. The preferredresins are highly crystalline (such as Fluon® CD123 supplied by ICI) butother resins can also be used in practicing this invention. (See U.S.Pat. Nos. 4,016,345 and 4,159,370.)

Processes which might appear to put the unsintered article under acompressive force, but do not achieve densification, can yield resultswhich are not consistent with the teachings of this invention. Forexample, U.S. Pat. No. 4,250,138 teaches a drawing step which mightappear to be consistent with the process described in Example 3 herein.Yet the opposite effect is acheived; i.e., finer structures are obtainedas indicated by increased EBP. U.S. Pat. Nos. 4,248,924 and 4,277,429teach a method of applying compressive forces to a film which mightappear to be consistent with the densification step described herein.Again, the opposite effect is achieved, i.e., the prior art process ispracticed to diminish the pore size of one side of a film relative tothe other side.

The conditions under which the densified article is stretched greatlyaffect the microstructure that is obtained. Higher rates of stretchingyield progressively finer microstructures, and there is the samequalitative interaction of rate of stretch and temperature duringstretching that is described in U.S. Pat. No. 3,953,566. Thus, densifiedunsintered articles can be stretched under conditions that will yieldproducts that are similar to prior art products, such as those taught inU.S. Pat. Nos. 4,187,390 and 3,962,153. The process of the presentinvention can also yield products with characteristics not heretoforeavailable. It is these latter materials which are sought to be uniquelyidentified by the values of parameters set forth in the claims.

The experiments that comprise the examples that follow demonstrate thatfor otherwise identical processing conditions, the addition of thedensification step produces coarser articles as compared to prior artarticles of comparable strength. The coarseness was characterized bypermeability, largest pore size, and dimensions of the nodes andfibrils. Materials produced with the densification step were seen tohave nodes that extended through the thickness of the article (i.e., inthe direction orthogonal to the direction(s) of stretch). This structurein a film or tape may result in higher peel strength, and/or highertensile strength in the thickness direction, and/or higher compressivestrength in the thickness direction.

Biaxially stretched films of the present invention have usefulness assurgical reinforcing membranes. Uniaxially stretched filaments of thepresent invention have usefulness as sutures and ligatures. Thesearticles are both strong and possess coarse microstructures. Coarsemicrostructures are desirable in medical applications because they allowcellular ingrowth and incorporation of body tissue. Films of the presentinvention can be useful in the manufacture of coaxial cables becausethey are both crush-resistant and porous. Films of the present inventionalso can be useful in applications demanding tensile strength in thethickness direction.

The following examples which disclose processes and products accordingto the present invention are illustrative only and are not intended tolimit the scope of the present invention.

EXAMPLE ONE Films That Are Uniaxially Stretched

PTFE resin (Fluon CD123, ICI) was paste-extruded as a film extrudate andcalendered. The calendered film was then dried to remove the extrusionaid. The properties of the dry, calendered film were as follows:thickness of about 0.016 inch, density of about 1.6 gm/cc, matrixtensile strength in the direction of extrusion of 1.6×10³ psi, andmatrix tensile strength in the transverse (width) direction of 0.6×10³psi. The dry, calendered extrudate was cut into approximately 4.5 inchby 4.5 inch specimens.

Some of the specimens were then densified by compression in a Carverpress that could be heated; the remaining specimens were leftundensified at the 1.6 gm/cc density level to serve as test controls.Gage blocks were used between the flat compression plates (and alongsidethe specimens) to control density by allowing densification only topredetermined thicknesses. In some cases, the gage blocks used werethinner than the thickness that was calculated to yield the desireddensity. These thinner blocks were required because some of the sampleswould regain some of their thickness after the compressive forces wereremoved. A range of densities was examined from 1.6 gm/cc ("control" -undensified) to values approaching the maximum achievable density), 2.3gm/cc. Densifications were carried out at temperatures from ambient (22°C.) to slightly above 300° C. The times to reach the desireddensification temperature and the times to reach the desireddensification at these temperatures were noted. The "control" pieceswere subjected to the same temperature and time conditions as were usedin densification. For convenience, two film samples were stackedtogether with a sheet of Kapton® polyimide film (DuPont) between them sothat two 4.5 inch by 4.5 inch samples of film could be simultaneouslydensified. The following steps were used to densify the dry PTFE film:

1. Carver press platens heated to specified temperature;

2. Film inserted between two flat steel plates along with Kaptonpolyimide film to serve as a release agent;

3. Gage blocks placed on perimeter of sheet. (Gage blocks not used fordensification to maximum density.);

4. Plates, with film between, placed inside press;

5. Platens closed until contact made;

6. Steel plates heated to desired temperature for densification;

7. Pressure applied and both steel plates slowly brought into contactwith the thickness gage blocks (or specimen, if gage blocks not used);

8. Pressure held for sufficient time to obtain desired densities;

9. Pressure released;

10. Materials densified at higher than ambient temperatures cooled inwater upon removal from the press.

The 4.5 inch by 4.5 inch specimens were weighed prior to thedensification step. Thickness measurements were taken at the fourcorners, at about one inch from each edge, and these four readings wereaveraged. The density was calculated by dividing the weight of thespecimen, by the area times the average thickness. This procedure yieldsa nominal density of the specimen, since the thickness of the specimenvaried due to local inconsistencies.

Materials (densified and undensified) were then stretched on apantograph in the longitudinal direction (i.e., the primary direction ofboth extrusion and calendering) to accomplish stretching. The pantographused was capable of stretching 4.5 inch by 4.5 inch samples of film toyield 4 inch by 16 inch samples for uniaxial stretching. (An extra 0.25inch length was required on each side of the specimens to accomodateclamping of the material in the machine.) The 4.5 inch by 4.5 inch filmwas gripped on each side by 13 actuated clamps, which could be movedapart uniformly on a scissor mechanism at constant velocity to stretchthe film. The film was heated to the desired temperature for stretching,by heater plates directly above and below the 4.5 inch by 4.5 inchsamples.

The stretch conditions were:

Temperature: approximately 300° C.

Stretch Ratio: 4:1 (300% increase in length)

Stretching Rate: approximately 400%/sec. (determined by dividing thepercent change in length by the duration of the stretching operation)

The stretched specimens were then restrained from shrinking, by placingthem on a pinframe, and immersed in a 370° C. salt bath for about 20seconds, thereby sintering the specimens.

Temperature did not appear to significantly affect the densificationprocess. Therefore, the data reported in Table 1 are averages of themeasurements obtained for given densities irrespective of thedensification temperature.

All data for matrix tensile strength, fibril length, and node width arereported for measurements made in the direction of stretch (which isalso the primary direction of extrusion and calendering). Break forceswere measured using specimens with a 1 inch gage length; the tensiletester cross-head speeds were 10 inches per minute. The density prior tostretching is listed as a single number and is the aforementionednominal value. The actual densities after densification varied due toexperimental variability and inevitable small measurement error. This,the individual measurements for the 1.63 gm/cc materials ranged from1.60 to 1.64 gm/cc. The individual measurements for the 1.83 gm/ccmaterials ranged from 1.75 to 1.85 gm/cc. The individual measurementsfor the 2.01 gm/cc materials ranged from 1.97 to 2.04 gm/cc. Theindividual measurements for the 2.27 gm/cc materials ranged from 2.19 to2.35 gm/cc. Therefore, the nominal range of 2.27 gm/cc includes themaximum obtainable densities.

FIGS. 4 and 5 present scanning electron micrographs of the surfaces offinal specimens (stretched and sintered) that had not been previouslydensified (nominal density of 1.63 gm/cc), and that had been previouslydensified to 2.27 gm/cc (nominal) prior to stretching, respectively. Themagnifications for the left and right sides of these two micrographs(FIGS. 4 and 5) are about 155 and 1550, respectively. These micrographsreadily demonstrate the difference in "coarseness" due to the effect ofdensification. FIGS. 6 and 7 present scanning electron micrographs ofthe cross-sections of the same two final specimens that had not beenpreviously densified, and that had been previously densified (2.27gm/cc), respectively. The magnifications for the left and right sides ofthe micrograph in FIG. 6 are about 152 and 1520, respectively. Themagnifications for the left and right sides of the micrograph in FIG. 7are about 147 and 1470, respectively. Again, the difference incoarseness is clear. These micrographs also demonstrate the differencein node height through the cross-section. The material produced by theprocess of this invention not only has nodes of greater heights comparedto the material that had not been densified, but a significant number ofthe nodes are seen to extend completely through the cross-section,unlike the case of the undensified material. These micrographs arerepresentative of all of the undensified control materials (1.63 gm/cc)and materials that had been densified at the density level, 2.27 gm/cc,regardless of densification temperature. The difference in coarseness asis apparent in FIGS. 4 and 5 and FIGS. 6 and 7 is reflected in thefibril length and node width measurements presented in Table 1. Thematerials made in accordance with the present invention had longeraverage fibril lengths and wider average node widths than materials thatwere not densified prior to stretching, but which had received otheridentical processing. Equally important, the data in Table 1 show theaverage matrix tensile strength in the stretch direction for allmaterials that had been densified prior to stretching to be at least ofthe same order of magnitude as the undensified, control materials. Thecombination of long fibril lengths, wide node widths, and high matrixtensile strengths relative to prior PTFE materials available fromconventional processes is surprising.

Returning to Table 1, the fibril lengths and widths of the nodes (in thedirection of stretching) were measured from scanning electronmicrographs of cross-sections of the stretched, sintered materials inorder to assess relative coarseness based upon the dimensions of themicrostructure. The fibril length and node width measurements utilizedscanning electron microscope pictures of about 150× magnification anddual magnification of 10× (about 1500× original), and the followingsteps:

1. SEM pictures were marked with two lines spaced approximately 24 mmapart, using a plexiglass fixture;

2. The fibril lengths were then determined using dividers to measure theinternodal spacing along the outside edge of the line starting at theupper left corner of the picture at the first distinct node spacing. Thedivider was then placed on a scale that accounted for the magnificationfactor and the lines were read to the nearest half micron, and valuesrecorded. This procedure was repeated for the next consecutive nodespacings along this line and each measurement recorded;

3. The procedure was repeated to measure node widths instead of spacebetween nodes, and the data recorded.

Examination of these data shows that the materials that were densifiedto a maximum degree, that is, in the 2.27 gm/cc range, (and subsequentlystretched and sintered) were significantly coarser than the othermaterials, as evidenced by the longer fibrils and wider nodes. The Table1 data are exemplary and show stretched materials that were densifiedless than 2.27 gm/cc prior to stretching had longer fibril lengths andwider node widths than the control pieces, but that the 2.27 gm/cc rangematerials had markedly coarser structures with no appreciable loss inmatrix tensile strength.

The data pertaining to Gurley number can characterize the coarseness ofthese materials. Lower values of this parameter indicate greaterpermeability of the structures. Permeability and, therefore, Gurleynumber measurements, are strongly dependent on path length. The use ofGurley number is an appropriate means of comparing the articlesdescribed in this example, however, since the materials were processedidentically except for the densification step. The data pertaining toethanol bubble point (EBP) also can characterize the coarseness ofmaterials. Lower values of this parameter indicate greater maximum poresize of the structures. Greater permeability, as well as larger poresize, quantify greater coarseness. The data in Table 1 demonstrate thematerials densified to about 1.83 and 2.01 gm/cc, upon stretching,exhibit lower values of the respective parameter than the 1.63 gm/cccontrol materials, and that the 2.27 gm/cc range materials had amarkedly higher permeability and larger pore sizes than the 1.63 gm/cccontrol materials. The final materials that had been densified in therange of 2.27 gm/cc did have markedly lower values for these parametersthan those materials that had been densified less.

The crushability test data in Table 1 demonstrate a macroscopicmanifestation of the coarse microstructure available through the presentinvention. In this test, the specimens were placed under a tensile loadby applying a 0.5 lb. force to the material in direction of stretching.A thickness measurement was taken which constituted the originalthickness. Next, an 18 oz. weight of 0.012 square inch area was appliedto the specimen for 0.5 minutes and the resulting thickness recordedwith the weight still applied. Percent crush, or crushability, isdefined as (t-C/t)(100%), in which "t" is the original thickness, and"C" is the thickness under load. Lower values of crushability,therefore, indicate a higher resistance to being crushed (i.e., a highercrush-resistance).

Again the most remarkable feature of these data is the difference incrushability between the materials densified to about 2.27 gm/cc,although the materials densified to lower densities did show improvedresistance to crushing over the undensified materials. The materialsdensified to about 2.27 gm/cc exhibited significantly greater resistanceto being crushed as evidenced by lower crushability.

The testing data indicate that densifying the dry, calendered extrudateto about 2.27 gm/cc or greater (i.e., the range of highest dendities)prior to stretching had an especially pronounced effect on the"coarseness" of the stretched, heat treated PTFE materials withoutdetracting from the matrix tensile strength.

Other samples were subsequently processed in essentially the same mannerin order to examine the utility of higher densification temperatures.The same ranges of density prior to stretching as used in theabove-mentioned experiments were examined for higher ranges ofdensification temperature. Consistent results were not obtained withmaterials subjected to elevated densification temperatures. Many of thefinal specimens were grossly non-uniform in appearance, unlike any ofthe final specimens that had not been subjected to these elevatedtemperatures prior to stretching. Some of the retained samples that hadbeen densified under identical conditions, but not stretched, weresubjected to differential scanning calorimetry analysis. Theidentification of reduced heats of fusion for these materials comparedto unprocessed resin indicated that the samples had been sintered tosome extent. The unintentional sintering was attributed, in part, tonon-uniformity of temperature across the plate. The important finding,however, is that partially or completely sintered materials, whetherdensified or not, cannot be stretched to yield uniform final materialsfor the above-mentioned stretch conditions.

The following conclusions can be reached from these tests:

1. The densification-stretching process yields high strength, coarsemicrostructure materials when extrudate is densified to 2.27 gm/cc. The2.27 gm/cc density actually refers to a range of densities obtained. Themaximum achievable density is included in this range.

2. The inclusion of a "dry" densification step (that is, with lubricantremoved from the extrudate) of any degree of densification prior tostretching does not compromise the matrix tensile strength of thestretched material.

3. Densifying dry extrudate to a density of 2.27 gm/cc prior tostretching results in a stretched material with a coarse structure,quantified by EBP, Gurley number, node width and fibril lengthmeasurements. By comparison, densifying to lower densities, or not atall, prior to stretching results in a stretched material with a finerstructure.

4. Densifying dry extrudate to a density of 2.27 gm/cc prior tostretching results in a more crush-resistant stretched material than ifthe dry extrudate is densified less, or not at all.

5. The degree of densification (as quantified by density measurements)has a very pronounced effect on the properties of the stretchedmaterial. The degree of densification essentially describes the salientfeature of the densification process provided that the material has notbeen sintered.

6. The influence of temperature is to serve as a process catalyst. Lesstime is required to reach the desired density in the densification stepfor higher densification temperatures. Increased temperature ofdensification may allow the use of lower compressive forces in order toachieve densification.

7. The preferred densification conditions are those that do not resultin any sintering of the dry extrudate.

                                      TABLE 1                                     __________________________________________________________________________    PROPERTIES OF THE FINAL SPECIMENS**                                                    Density Prior To Stretching                                                   1.63 ± .01 gm/cc*                                                                    1.83 ± .02 gm/cc                                                                    2.01 ± .02 gm/cc                                                                    2.27 ± .05 gm/cc                      __________________________________________________________________________    Thickness                                                                              .0119 ± .0002                                                                        .0118 ± .0002                                                                       .0114 ± .0002                                                                       .0112 ± .0005                         (inch)                                                                        Density  .56 ± .02                                                                            .57 ± .02                                                                           .59 ± .02                                                                           .58 ± .04                             (gm/cc)                                                                       Matrix Tensile                                                                         15,600 ± 700                                                                         15,700 ± 900                                                                        15,900 ± 600                                                                        16,500 ± 100                          Strength                                                                      (psi)                                                                         Fibril Length                                                                          4. ± 1.                                                                              5. ± 1.                                                                             5. ± 1.                                                                             23. ± 4.                              (microns)                                                                     Node Width                                                                             3. ± 1.                                                                              3. ± 1.                                                                             4. ± 1.                                                                             15. ± 3.                              (microns)                                                                     EBP      7.8 ± .5                                                                             6.9 ± .4                                                                            6.4 ± .6                                                                            2.5 ± 1.0                             (psi)                                                                         Gurley Number                                                                          27.5 ± 3.8                                                                           23.8 ± 3.5                                                                          19.4 ± 3.5                                                                          6.5 ± 2.4                             (seconds)                                                                     Crushability                                                                           15. ±  1.                                                                            14. ± 2.                                                                            14. ± 2.                                                                            9. ± 2.                               (%)                                                                           Coarseness Index                                                                       .07       .08      .09      .23                                      [(gm/cc)/psi]                                                                 __________________________________________________________________________     **All values are rounded. The values presented are the means ± one         standard deviation calculated from the mean values for each specimen          produced at each density level prior to stretching.                           *Control with no densification step.                                     

EXAMPLE TWO Films That Are Biaxially Stretched

Four other 4.5 inch by 4.5 inch samples of film of the type described inthe first paragraph of Example One above were stretched in thepantograph machine. In this case, three samples were densified in theCarver press at temperatures of about 300° C. and a fourth sample wassubjected to the same thermal conditions, but not densified. Theundensified material served as a test control. The materials weredensified in essentially the same manner as described in the thirdparagraph of Example One.

All four samples were stretched simultaneously in two directions atright angles to each other in the pantograph machine (described inExample One), 100% in each direction. Thus, the surface area of thestretched film was four times greater than the surface area of theoriginal film. The film temperature was about 300° C. at the start ofthe stretching operation. Stretching rates of about 130% per second ineach direction were used. Stretching rate was determined by dividing thepercent change in length by the duration of the stretching operation.(The clamps of the pantograph moved apart at constant velocity.) Thestretched specimens were then restrained from shrinking by placing themon a pinframe, removed from the pantograph machine clamps, and immersedin a 370° C. salt bath for about 20 seconds, thereby sintering thespecimens. The specimens were then cooled in water to yield the finalspecimens.

The data in Table 2 show the effects of this invention. FIGS. 8 and 9present scanning electron micrographs of the surfaces of the controlmaterial (1.61 gm/cc) and the material that had been densified to 2.25gm/cc, respectively. The magnifications for the left and right sides ofthese micrographs in FIGS. 8 and 9 are about 150 and 1500, respectively.The relative coarseness of the material that had been densified to 2.25gm/cc is readily apparent. These figures demonstrate the structuraldifferences due to the invention that result in the difference inethanol bubble points as indicated in Table 2. The micrograph in FIG. 9is representative of the structure that resulted due to the inclusion ofthe densification step. The final material is not completely uniform,however, and some regions are not seen to be as coarse with regard tothe dimensions of the microstructure as other regions of the samematerial. This non-uniformity is attributed to local inconsistenciesduring the densification.

The data in Table 2 show that the material that was densified the mostprior to stretching was far more crush-resistant than the materials thatwere densified less or not at all. Four additional samples were producedfrom the same raw materials using the same processes in order to furtherexamine the benefits of the present invention with respect tocrush-resistance. The same range of densities prior to stretching wasexamined. These samples, unlike those whose data appear in Table 2, werenot sintered subsequent to stretching. The data for these materials thatwere not sintered appear in Table 3. The crushability for the stretchedmaterials with pre-stretching densities of 1.63, 1.89, 2.06, and 2.29gm/cc were 30.1, 19.7, 10.2, and 3.6%, respectively, showing that thosematerials that were densified the most produced the most crush-resistantfinal products. Comparing the data for the sintered and unsinteredmaterials that were not densified indicates that sintering serves todecrease crushability for undensified materials (from 30.1% to 14.6%, inthis case). The material that was densified the most but not sinteredwas still far more crush-resistant (a crushability of 3.6%) than theundensified material that was sintered (which had a crushability of14.6%).

Break forces were measured using specimens with a 1 inch gage length;the tensile tester cross-head speed was 10 inches per minute. Thelongitudinal direction is the primary direction of extrusion andcalendering. The transverse direction is orthogonal to the longitudinaldirection, in the plane of stretch.

                  TABLE 2                                                         ______________________________________                                        PROPERTIES OF THE FINAL SPECIMENS**                                                   Density Prior To Stretching                                                   1.61      1.83     2.02     2.25                                              gm/cc*    gm/cc    gm/cc    gm/cc                                     ______________________________________                                        Thickness .0111       .0109    .0105  .0122                                   (inch)                                                                        Density   .57         .54      .65    .54                                     (gm/cc)                                                                       Ethanol   9.8         7.0      3.6    1.2                                     Bubble                                                                        Point                                                                         (psi)                                                                         Longitudinal                                                                            10,100      8,300    7,000  6,500                                   Matrix                                                                        Tensile                                                                       Strength                                                                      (psi)                                                                         Transverse                                                                              10,200      11,200   8,600  6,400                                   Matrix Tensile                                                                Strength                                                                      (psi)                                                                         Coarseness Index                                                                        .06         .08      .18    .45                                     [(gm/cc)/psi]                                                                 Crushability                                                                            14.6        17.0     16.6   4.2                                     (%)                                                                           ______________________________________                                         **All values are rounded.                                                     *Control with no densification step.                                     

                  TABLE 3                                                         ______________________________________                                        PROPERTIES OF THE FINAL SPECIMENS THAT                                        WERE NOT SINTERED AFTER STRETCHING**                                                   Density Prior To Stretching                                                   1.63     1.89     2.06     2.29                                               gm/cc*   gm/cc    gm/cc    gm/cc                                     ______________________________________                                        Thickness  .0146      .0130    .0120  .0116                                   (inch)                                                                        Density    .58        .63      .61    .72                                     (gm/cc)                                                                       Ethanol    14.2       8.1      4.8    3.1                                     Bubble                                                                        Point                                                                         (psi)                                                                         Longitudinal                                                                             4,400      4,300    4,800  3,800                                   Matrix                                                                        Tensile                                                                       Strength                                                                      (psi)                                                                         Transverse 2,400      2,200    2,800  2,400                                   Matrix Tensile                                                                Strength                                                                      (psi)                                                                         Coarseness Index                                                                         .04        .08      .13    .23                                     [(gm/cc)/psi]                                                                 Crushability                                                                             30.1       19.7     10.2   3.6                                     (%)                                                                           ______________________________________                                         **All values are rounded.                                                     *Control with no densification step.                                     

EXAMPLE 3 Filaments That Are Uniaxially Stretched Part A

Part A illustrates the effect a densification die can have on themicrostructure of a uniaxially stretched filament. The processing of thetwo finished filaments described herein was adJusted to yield materialswith equivalent diameters, densities, and matrix tensile strengths.

PTFE dispersion powder ("Fluon CD-123" resin produced by ICI America)was blended with 130 cc of "Isopar M" odorless solvent (produced byExxon Corporation) per pound of PTFE, compressed into a pellet, andextruded into a 0.106 inch diameter filament in a ram extruder having a95.1 reduction ratio in cross-sectional area from the pellet to theextruded filament.

The Isopar M was evaporated from a sample of the extruded filament. Thedensity of this sample was about 1.49 gm/cc, and its matrix tensilestrength was about 900 pounds per square inch.

The extruded filament still containing Isopar M was immersed in acontainer of Isopar M at 60° C., and stretched nine-fold (800%) betweencapstans with an output velocity of about 86.4 ft/min. These capstanshad a diameter of about 2.8 inches and a center-to-center distance ofabout 4.5 inches. The diameter of the filament was reduced from about0.106 inch to about 0.039 inch by this stretching. The Isopar M wasremoved from this stretched material. The density of the stretchedfilament was about 1.3 gm/cc, and the matrix tensile strength was about5,400 pounds per square inch.

The stretched filament, from which the Isopar M had been removed, wasthen pulled through a circular densification die heated to 300° C. Theoutput velocity of the material exiting the die was about 7.2 ft/minute.The opening in the die tapered at a 10° included angle from about 0.050inch diameter to 0.030 inch diameter, and was then constant for a landlength of about 0.030 inches.

The die diameter of 0.030 inches was chosen on the basis of twoassumptions:

1. It was desirable to densify the stretched filament to approximately2.2 gm/cc.

2. There would be no weight/meter change of the stretched rod as itunderwent densification.

Using these assumptions, die diameter was calculated to represent thereduction in cross-sectional area necessary to increase the density ofthe stretched rod to about 2.2 gm/cc. In the specific case of the A-16filament, that calculation was worked as follows: ##EQU1## D₁ =initialdiameter of stretched rod in inches D₂ =die diameter in inches

ρ₁ =initial density of stretched rod in gm/cc

ρ₂ =nominal value of void-free PTFE as 2.2 gm/cc

Removing a piece of filament from the die, by halting the densificationprocess and pulling the material back through the entrance of the die,showed that when the stretched filament was pulled through thedensification die it developed a translucent segment characteristic ofPTFE having a density of about 2.2 gm/cc. This segment corresponded tothe 0.030 inch land length section in the die immediately following the10° included angle transition (see FIG. 10).

As the material exited the die, however, it once again developed a whiteappearance characteristic of PTFE having a density less than about 2.2gm/cc. This is because the force necessary to pull the stretchedfilament through the die is sufficient to cause some stretching of thematerial after it exits the die. This was confirmed by measuring theweight/meter of the material pre- and post-die. A decrease inweight/meter was noted in the material post-die, indicating stretchingtook place. Subsequent experimental work demonstrates that die diametersboth greater and smaller than 0.030 inches can also effect the desiredchange in microstructure. The important consideration in choosing a diediameter is that it changes the cross-sectional area of the stretchedrod so as to achieve a material density in the die greater than or equalto about 2.0 gm/cc. There is an increasing effect on structure withincreasing densification.

The stretched filament, which had been pulled through the die, was thenheated in a 300° C. oven and further stretched seven-fold (600%), froman intial length of about 7.2 inches, in a batch manner with a constantvelocity of about 37 ft/min.

Finally, the filament was restrained from shrinking and heated in a 367°C. oven for 30 seconds.

As described in Table 4, the filament (A-16) from the final heattreatment had a density of about 0.4 gm/cc, a diameter of about 0.022inch, and a matrix tensile strength of about 49,000 pounds per squareinch. The structure was comprised of nodes of apparently solid PTFEinterconnected by fibrils. The average fibril length was about 120microns, the average node width about 17 microns (measured in thedirection of stretch), and the average node height about 102 microns(measured orthogonal to the direction of stretch). The filamentunderwent a total stretch ratio of 79:1 from the extrudate stage. Thiswas calculated by dividing the dried filament extrudate weight/meter bythe finished filament weight/meter.

One end of a length of the A-16 filament was heated, densified and thenswaged onto a standard 0.022 inch diameter surgical needle, making aprototype suture with matching needle and thread diameters. Thisneedle/thread combination is not currently available in the marketplaceand has the potential advantage of reducing suture line bleeding invascular anastomoses. This material was sewn into the tissue of a guineapig and harvested after 30 days. Fibroblast cells had penetrated intothe structure of the suture, and substantial collagen was formedthroughout the inner structure of the suture (see FIG. 11A). Also, thesuture became well embedded in the tissue. These attributes, combinedwith the material's strength and ease of handling, should make it usefulas a suture.

Another material (3-1-3) was manufactured using a process similar tothat described above with the major exception that the stretchedfilament was not pulled through a densification die. Minor processingchanges were necessary to achieve this equivalence. Specifically, theextruded filament had a diameter of about 0.090, a matrix tensile ofabout 1200 psi and it underwent a total stretch of 52:1 through theprocess. Table 4 demonstrates that this material has a diameter, matrixtensile strength, and density nearly indentical to that of the materialwhich had been pulled through the die. When implanted in guinea pigsthis material (3-1-3) permitted only minimal collagen penetration (seeFIG. 11B).

As illustrated by the pictures in FIGS. 12A and 12B and information inTable 4, these materials have vastly different microstructures. The A-16material had much longer fibril lengths, and nodes where itsheight/width (H/W) ratio was substantially greater than in theundensified (3-1-3) material.

For materials with matrix tensile strengths greater than about 15,000psi this node relationship of H/W is unique. Previously, only thosematerials with matrix tensile strengths less than about 15,000 psi had anode H/W ratio greater than or equal to 3. Conversely, when prior artmatrix tensile strengths get above about 15,000 psi, the node H/W ratiodrops below about 3. The only materials with matrix tensile strengthsgreater than or equal to about 15,000 psi having a node H/W ratiogreater than or equal to about 3 are those materials which undergo adensification step prior to stretching, where the densificationincreases the specific gravity of the material to greater than or equalto about 2.0 g/cc. It appears from this example, that densificationprior to final stretching may yield filaments having longer fibrillengths than would be achieved with similar amounts of stretch in aprocess not including a densification step.

Part B

The following example further illustrates the effect of thedensification die on microstructure. No attempt was made to matchcharacteristics of the finished filaments as in Part A. Final stretchingof both materials described herein was adjusted so that they underwentidentical amounts of stretch from the extrusion stage. This was done toinvestigate the effects of densification on equivalently stretchedpieces of material from the same extrusion batch.

PTFE dispersion powder ("Fluon CD-123" resin produced by ICI America)was blended with 130 cc of "Isopar M" odorless solvent (produced byExxon Corporation) per pound of PTFE, compressed into a pellet, andextruded into a 0.108 inch diameter rod in a ram extruder having a 153:1reduction ratio in cross-sectional area from the pellet to the extrudedfilament.

The Isopar M was evaporated from a sample of the extruded filament. Thedensity of this sample was 1.48 gm/cc and its matrix tensile strengthwas about 1,700 pounds per square inch.

The extruded filament still containing Isopar M was immersed in acontainer of Isopar M at 60° C. and stretched seven-fold (600%) betweencapstans with an output velocity of about 57.6 ft/min. These capstanshad a diameter of about 2.8 inches and a center-to-center distance ofabout 4.5 inches. The diameter of the filament was reduced from about0.108 inch to about 0.049 inch by this stretching. The Isopar M wasremoved from this stretched material. The density of the stretchedfilament was 1.02 g/cc and the matrix tensile strength was about 7,900pounds per square inch.

At this point the stretched filament was divided into two separate lotsfor further processing. Lot 661 was pulled through a densification die,while Lot 665 was not.

The stretched filament (Lot 661), from which the Isopar M had beenremoved, was then pulled through a circular densification die heated to300° C. The output velocity of the material exiting the die was about1.9 ft/minute. The opening in the die tapered at a 10° included anglefrom about 0.075 inch diameter to 0.026 inch diameter, and was thenconstant for a land length of about 0.026 inch.

The stretched filament (Lot 661), which had been pulled through the die,was then heated to 300° C. and further stretched 4.5 fold (350%) betweencapstans with an output velocity of about 13 ft/min. These capstans hada diameter of about 2.8 inches and a center-to-center distance of about24 inches.

The stretched filament (Lot 665), which had not been pulled through thedie, was heated in a 300° C. oven and further stretched eight-fold(700%) between the same capstan setup just described, using an outputvelocity of about 11.5 ft/min.

Finally, both rods (filaments) were restrained from shrinking and heatedin a 362° C. oven for 60 seconds.

As illustrated by the pictures in FIGS. 13A and 13B and information inTable 5, these two types of material had vastly differentmicrostructures. Lot 661 had much longer fibril lengths, and nodes wherethe H/W ratio was substantially larger than in the undensified (Lot 665)material. This example clearly illustrates that filaments which aredensified prior to final stretching have much longer fibril lengths thando undensified filaments, when both materials undergo equivalent amountsof stretch.

These examples, Parts A and B, demonstrate that densifying a stretchedfilament through the use of a densification die can result in a highstrength material with a unique microstructure upon further stretching.The important aspect of the invention is that the stretched filament wasdensified to greater than or equal to about 2.0 g/cc prior to additionalstretching.

                  TABLE 4                                                         ______________________________________                                        Finished Filament Characteristics                                                            A-16    3-1-3                                                                 (Die)   (No Die)                                               ______________________________________                                        Density (g/cc)   .4        .5                                                 Matrix Tensile (psi)                                                                           49,000    49,000                                             Diameter (inches)                                                                              .022      .022                                               Node Width (microns)                                                                            17        9                                                 Node Height (microns)                                                                          102       16                                                 H/W Ratio        6         1.8                                                Fibril Length (microns)                                                                        120       32                                                 TOTAL STRETCH RATIO                                                                            79:1      52:1                                               TISSUE INGROWTH  collagen  minimum collagen                                                    throughout                                                                              infiltration at                                                     interstices                                                                             30 days                                                             at 30 days                                                   ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Finished Filament Characteristics                                                               661   665                                                                     (Die) (No Die)                                              ______________________________________                                        Density (g/cc)      .6      .5                                                Matrix Tensile (psi)                                                                              55,000  64,000                                            Diameter (inches)   .022    .025                                              Node Width (microns)                                                                              11      6                                                 Node Height (microns)                                                                             79      3                                                 H/W Ratio           7.2     .5                                                Fibril Length (microns)                                                                           74      16                                                TOTAL STRETCH RATIO 58:1    57:1                                              ______________________________________                                    

To obtain the above listed densities, material volumes were calculatedfrom diameter and length measurements, and this volume was divided intothe weight of the material. Density calculations are accurate to twodecimal places. Matrix tensile values were calculated as described aboveand are accurate to one decimal place. Diameters were measured using anon-contacting laser micrometer. The values listed represent the averagediameter of several feet of material and are accurate to four decimalplaces.

To obtain node widths, node heights, and fibril lengths, pictures with a200:1 magnification were used. The pictures were taken on a scanningelectron microscope and a Nikon Biophot (Brightfield Microscope).Measurements were taken with millimeter calipers and then converted tomicrons. Measurements were chosen (4 to 5 measurements per picture for agiven material type) by randomly drawing two horizontal lines on eachpicture approximately 1 inch apart. Five consecutive measurements werethen taken, starting at the left margin. After obtaining 20measurements, mean valves were calculated. Node width, node height, andfibril length values are accurate to one decimal place. Total stretchratio was calculated by dividing the dried filament extrudateweight/meter by the finished filament weight/meter. Ratios calculatedare accurate to one decimal place.

What is claimed is:
 1. A porous material consisting essentially ofpolytetrafluroroethylene polymer, which material has a microstructurecharacterized by nodes interconnected by fibrils and as measured alongat least one direction, has a combination of average matrix tensilestrength greater than about 15,000 psi and an average node height/widthratio greater than about
 3. 2. A porous material consisting essentiallyof polytetrafluoroethylene polymer, which material has a microstructurecharacterized by nodes interconnected by fibrils and as measured alongat least one direction, has a combination of average matrix tensilestrength greater than about 40,000 psi and an average node height/widthratio greater than about
 3. 3. A porous material consisting essentiallyof polytetrafluoroethylene polymer, which material has a microstructurecharacterized by nodes interconnected by fibrils and as measured alongat least one direction, has a combination of average matrix tensilestrength greater than about 15,000 psi and an average node height/widthratio greater than about
 5. 4. A porous material consisting essentiallyof polytetrafluoroethylene polymer, which material has a microstructurecharacterized by nodes interconnected by fibrils and as measured alongat least one direction, has a combination of average matrix tensilestrength greater than about 40,000 psi and an average node height/widthratio greater than about
 5. 5. The porous material of claim 1-3 or 4having average fibril length of greater than about 15 microns.
 6. Theporous material of claim 1-3 or 4 having average fibril length ofgreater than about 50 microns.
 7. A porous material consistingessentially of polytetrafluoroethylene polymer, which material has amicrostructure characterized by nodes interconnected by fibrils and asmeasured along at least one direction, has a combination of averagematrix tensile strength of about 49,000 psi, average fibril length ofabout 120 microns, and an average node height/width ratio of about
 6. 8.A porous material consisting essentially of polytetrafluoroethylenepolymer, which material has a microstructure characterized by nodesinterconnected by fibrils and as measured along at least one direction,has a combination of average matrix tensile strength of about 55,000psi, average fibril length of about 74 microns, and an average nodeheight/width ratio of about
 7. 9. The material in claims 1-4, 7 or 8 infilament form.
 10. A porous material consisting essentially ofpolytetrafluoroethylene polymer, which material is characterized bynodes interconnected by fibrils, said material having a matrix tensilestrength greater than or equal to about 3,000 psi and less than or equalto about 25,000 psi and which has a corresponding coarseness indexgreater than or equal to the value on a line connecting the points A, B,C, and D in FIG. 3 and defined in the following table:

    ______________________________________                                        MATRIX TENSILE                                                                STRENGTH          COARSENESS INDEX                                            psi               (gm/cc)/psi                                                 ______________________________________                                        A       3,000         0.40                                                    B      12,000         0.40                                                    C      16,000         0.20                                                    D      25,000         0.20                                                    ______________________________________                                    


11. Material as in claim 10 wherein point C and point D have acoarseness index of 0.23.
 12. Material as in claim 10 wherein point Cand point D have a coarseness index of 0.40.
 13. Material as in claim 10wherein points A, B, C, and D have a coarseness index of 0.45.
 14. Thematerial in claims 10-12 or 13 in film form.
 15. A film of the materialas in claim 10-12 or 13 having a crushability less than about 10%.
 16. Afilm of the material as in claim 10-12 or 13 having a crushability lessthan about 8%.
 17. A film of the material as in claim 10-12 or 13 havinga crushability less than about 5%.
 18. A porous material consistingessentially of polytetrafluoroethylene polymer, which material has amicrostructure characterized by nodes interconnected by fibrils and hasan ethanol bubble point of less than or equal to about 4.0, wherein saidfibrils include first fibrils oriented substantially perpendicular tosecond fibrils, and wherein the ratio of the matrix tensile strength asmeasured along the first fibril direction to the matrix tensile strengthmeasured along the second fibril direction is between about 0.4 and 2.5,and the matrix tensile strength in the weaker direction is greater thanor equal to about 3000 psi.
 19. The material in claim 18 wherein theethanol bubble point is less than about 3.0
 20. The material in claim 18wherein the ethanol bubble point is about 1.2.
 21. The material as inclaim 18, 19 or 20 wherein the matrix tensile strength ratio is betweenabout 0.5 and 2.0.
 22. The material as in claims 15, 16 or 17 whereinthe matrix tensile strength is between about 0.67 and 1.5.
 23. A film ofthe material as in claims 10-13, 18, 19 or 20 wherein a significantnumber of the nodes extend across the film thickness.